Mixtures of polarised fermions of two different masses can form weakly-bound clusters, such as dimers and trimers, that are universally described by the scattering length between the heavy and light fermions. We use the resonating group method to investigate the low-energy scattering processes involving dimers or trimers. The method reproduces approximately the known particle-dimer and dimer-dimer scattering lengths. We use it to estimate the trimer-trimer scattering length, which is presently unknown, and find it to be positive.